Radiation detection approaches exist that employ photo sensors incorporating a microcell (e.g., a single photon avalanche diodes (SPAD)) operating in Geiger mode. Certain of these approaches have been implemented in large area devices, such as may be used in nuclear detectors. A readout pixel can be made up of an array of microcells, where each individual microcell can be connected to a readout network via a quenching resistor exhibiting resistance between 100 kΩ to 1 MΩ. When a bias voltage applied to the SiPM is above breakdown, a detected photon generates an avalanche, the APD capacitance discharges to a breakdown voltage and the recharging current creates a signal.
Typically, the pulse shape associated with a single photo electron (SPE) signal has a fast rise time, followed by a long fall time. When detecting fast light pulse (e.g., on the order of tens of nanoseconds) such signals are aggregated across the numerous microcells forming a pixel of a silicon photomultiplier (SiPM) device. The resulting pulse shape of the summed signal has a slow rise time (e.g., in the tens of nanoseconds) due to the convolution of single microcell responses with detected light pulse. Therefore, it is difficult to achieve good timing resolution with these devices due to the slow rise time of the aggregated signal for a given light pulse.
To address the problem of slow rise times, digital SiPMs (dSiPM) can be employed. In this approach, special electronic circuitry for each microcell (e.g., a SPAD) is produced on the same silicon wafer using a complementary metal-oxide semiconductor (CMOS) process. The function of this circuitry is to detect avalanche events and to actively quench the microcell. Each circuitry has a memory element (such as a 1 or more bit element). A special network tree is used to collect time stamps from all the microcells. To get the information of the number of detected photons per event a special read out cycle is executed, which requires a special digital controller for each dSiPM. Such an approach is undesirably complex.
Analog SiPM can have the pixel outputs wire-summed and bonded-out by wires attached to the wafer, or by using short vertical interconnects implemented in Through-Silicon-Via (TSV) technology. Microcells can be connected by traces, and typically one or a few pads per array of microcells (pixel) can be used as output (wire bounds or TSV). An analog SiPM typically requires a front-end to buffer (and/or amplify) the signal from the SiPM for further processing. Digital SIPM technology has analog and/or digital electronics built-in to the microcell to produce a digital output pulse. The microcells of a dSiPM communicate with an external controller having typically high clock speeds.
For both analog and digital SiPM devices, due to the variation of signal trace length the propagation delay varies from microcell to microcell, which degrades timing. For analog SiPM devices the signal pulse rise time degrades due to both on- and off-chip parasitics (e.g., bond wire inductance, and PCB signal trace affects) and limited driving capability of a microcell.
Due to the difference in the actual geometry of traces that connect microcells to readout electronics, there can be a significant variation of time delay across pixels. Long circuit traces can also degrade pulse shapes due to propagation dispersion. Attempting to equalize trace length can significantly increase parasitics, and degrade signal pulse shape due to the limited driving capability of the microcell.
A complicated trigger network and an on-chip time-digital-converter (TDC) can be fabricated on the same wafer as the dSiPM to readout timing. The TDC outputs a digitized timing stamp to an external controller (e.g., a field programmable gate array (FPGA)). Photons can also be sensed using a hybrid SiPM (HSiPM), which takes advantage of both the photon counting capability of each microcell and the simplicity of analog SiPM.
In HSiPM, each microcell has built-in active electronics. A firing microcell generates a predefined digital pulse. The digital pulses from all the firing microcells are summed in an analog fashion and output to external electronics for timing and energy readout. Since the microcells in HSiPM generate a digital pulse for every single avalanche, one may use this information for energy readout, instead of digitizing the summed digital pulses.
The conventional method to get the best timing in analog SiPM is to adjust the threshold of the discriminators. Due to the noise in the system, it is impractical to set the threshold low enough to discriminate the vent on the level of several photons (e.g., one photon, two photons) which would provide the best timing. A precise triggering level is difficult to determine due to the limitation of trigger logic trace and the impact parasitics have on signal quality.